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Operating System ServicesOne set of operating-system services provides functions that are helpfulto the user:

User interface - Almost all operating systems have a userinterface (UI)

Varies between Command-Line (CLI) , Graphics User Interface(GUI), Batch

Program execution - The system must be able to load aprogram into memory and to run that program, end execution,either normally or abnormally (indicating error)I/O operations - A running program may require I/O, which mayinvolve a file or an I/O device

File-system manipulation - The file system is of particularinterest. Obviously, programs need to read and write files anddirectories, create and delete them, search them, list fileInformation, permission management.

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Operating System Services (Cont)

One set of operating-system services provides functions that arehelpful to the user (Cont):

Communications Processes may exchange information,on the same computer or between computers over anetwork

Communications may be via shared memory or throughmessage passing (packets moved by the OS)

Error detection OS needs to be constantly aware ofpossible errors

May occur in the CPU and memory hardware, in I/O devices, inuser programFor each type of error, OS should take the appropriate action toensure correct and consistent computingDebugging facilities can greatly enhance the users and

programmers abilities to efficiently use the system

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Operating System Services (Cont)

Another set of OS functions exists for ensuring the efficient operation of thesystem itself via resource sharing

Resource allocation - When multiple users or multiple jobs runningconcurrently, resources must be allocated to each of them

Many types of resources - Some (such as CPU cycles, main memory,and file storage) may have special allocation code, others (such as I/Odevices) may have general request and release code

Accounting - To keep track of which users use how much and what kinds ofcomputer resourcesProtection and security - The owners of information stored in a multiuser ornetworked computer system may want to control use of that information,concurrent processes should not interfere with each other

Protection involves ensuring that all access to system resources iscontrolledSecurity of the system from outsiders requires user authentication,extends to defending external I/O devices from invalid access attemptsIf a system is to be protected and secure, precautions must be institutedthroughout it. A chain is only as strong as its weakest link.

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User Operating System Interface - CLI

Command Line Interface (CLI) or commandinterpreter allows direct command entry

Sometimes implemented in kernel, sometimes bysystems program

Sometimes multiple flavors implemented shellsPrimarily fetches a command from user and executes itSometimes commands built-in, sometimes just names ofprograms

If the latter, adding new features doesnt require shellmodification

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User Operating System Interface - GUI

User-friendly desktop metaphor interfaceUsually mouse, keyboard, and monitorIcons represent files, programs, actions, etcVarious mouse buttons over objects in the interface cause

various actions (provide information, options, executefunction, open directory (known as a folder )Invented at Xerox PARC

Many systems now include both CLI and GUI

interfacesMicrosoft Windows is GUI with CLI command shell Apple Mac OS X as Aqua GUI interface with UNIXkernel underneath and shells available

Solaris is CLI with optional GUI interfaces (Java Desktop,KDE)

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Example of Standard APIConsider the ReadFile() function in the

Win32 API a function for reading from a file

A description of the parameters passed to ReadFile()HANDLE file the file to be read

LPVOID buffer a buffer where the data will be read into and written fromDWORD bytesToRead the number of bytes to be read into the bufferLPDWORD bytesRead the number of bytes read during the last readLPOVERLAPPED ovl indicates if overlapped I/O is being used

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System Call Parameter PassingOften, more information is required than simplyidentity of desired system call

Exact type and amount of information vary according toOS and call

Three general methods used to pass parameters to

the OSSimplest: pass the parameters in registers In some cases, may be more parameters than registers

Parameters stored in a block, or table, in memory, andaddress of block passed as a parameter in a register

This approach taken by Linux and SolarisParameters placed, or pushed, onto the stack by theprogram and popped off the stack by the operatingsystemBlock and stack methods do not limit the number orlength of parameters being passed

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Types of System Calls

Process controlFile managementDevice managementInformation maintenanceCommunicationsProtection

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System Programs

System programs provide a convenientenvironment for program development andexecution. The can be divided into:

File manipulationStatus informationFile modificationProgramming language supportProgram loading and executionCommunications

Application programs

Most users view of the operation system isdefined by system programs, not the actualsystem calls

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System ProgramsProvide a convenient environment for program development and execution

Some of them are simply user interfaces to system calls; others are considerablymore complex

File management - Create, delete, copy, rename, print, dump, list, and generallymanipulate files and directories

Status information

Some ask the system for info - date, time, amount of available memory, disk space,number of users

Others provide detailed performance, logging, and debugging informationTypically, these programs format and print the output to the terminal or other outputdevices

Some systems implement a registry - used to store and retrieve configuration

information

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System Programs (contd) File modification

Text editors to create and modify filesSpecial commands to search contents of files or performtransformations of the text

Programming-language support - Compilers, assemblers,

debuggers and interpreters sometimes providedProgram loading and execution- Absolute loaders,relocatable loaders, linkage editors, and overlay-loaders,debugging systems for higher-level and machinelanguageCommunications - Provide the mechanism for creatingvirtual connections among processes, users, andcomputer systems

Allow users to send messages to one anothers screens,browse web pages, send electronic-mail messages, log inremotely, transfer files from one machine to another

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Operating System Design and Implementation

Design and Implementation of OS not solvable , butsome approaches have proven successfulInternal structure of different Operating Systems canvary widelyStart by defining goals and specifications

Affected by choice of hardware, type of systemUser goals and System goals

User goals operating system should be convenient touse, easy to learn, reliable, safe, and fastSystem goals operating system should be easy todesign, implement, and maintain, as well as flexible,reliable, error-free, and efficient

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Operating System Design and Implementation (Cont)

Important principle to separatePolicy: What will be done? Mechanism: How to do it?Mechanisms determine how to do something,

policies decide what will be doneThe separation of policy from mechanism is a veryimportant principle, it allows maximum flexibility if policydecisions are to be changed later

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Simple Structure

MS-DOS written to provide the most functionality inthe least spaceNot divided into modules

Although MS-DOS has some structure, its interfaces andlevels of functionality are not well separated

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Layered Approach

The operating system is divided into a number oflayers (levels), each built on top of lower layers. Thebottom layer (layer 0), is the hardware; the highest(layer N) is the user interface.

With modularity, layers are selected such that eachuses functions (operations) and services of onlylower-level layers

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UNIX

UNIX limited by hardware functionality, theoriginal UNIX operating system had limitedstructuring. The UNIX OS consists of twoseparable parts

Systems programsThe kernel

Consists of everything below the system-call interface andabove the physical hardware

Provides the file system, CPU scheduling, memorymanagement, and other operating-system functions; alarge number of functions for one level

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Microkernel System Structure

Moves as much from the kernel into user space Communication takes place between user modulesusing message passingBenefits:

Easier to extend a microkernelEasier to port the operating system to new architecturesMore reliable (less code is running in kernel mode)More secure

Detriments:Performance overhead of user space to kernel spacecommunication

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Modules

Most modern operating systems implement kernelmodulesUses object-oriented approachEach core component is separate

Each talks to the others over known interfacesEach is loadable as needed within the kernel

Overall, similar to layers but with more flexible

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Virtual Machines

A virtual machine takes the layeredapproach to its logical conclusion. It treatshardware and the operating system kernelas though they were all hardware

A virtual machine provides an interfaceidentical to the underlying bare hardwareThe operating system host creates theillusion that a process has its ownprocessor and (virtual memory)Each guest provided with a (virtual) copy ofunderlying computer

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Virtual Machines History and Benefits

First appeared commercially in IBM mainframes in 1972Fundamentally, multiple execution environments(different operating systems) can share the samehardwareProtect from each other

Some sharing of file can be permitted, controlledCommutate with each other, other physical systems vianetworkingUseful for development, testing

Consolidation of many low-resource use systems ontofewer busier systemsOpen Virtual Machine Format , standard format of virtualmachines, allows a VM to run within many different virtualmachine (host) platforms

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Para-virtualization

Presents guest with system similar but not identicalto hardwareGuest must be modified to run on paravirtualizedhardwareF

Guest can be an OS, or in the case of Solaris 10applications running in containers

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Operating-System DebuggingDebugging is finding and fixing errors, or bugsOSes generate log files containing error informationFailure of an application can generate core dump file capturingmemory of the processOperating system failure can generate crash dump filecontaining kernel memoryBeyond crashes, performance tuning can optimize systemperformanceKernighans Law: Debugging is twice as hard as writing thecode in the rst place. Therefore, if you write the code ascleverly as possible, you are, by denition, not smart enoughto debug it. DTrace tool in Solaris, FreeBSD, Mac OS X allows liveinstrumentation on production systems

Probes fire when code is executed, capturing state data andsending it to consumers of those probes

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Operating System Generation

Operating systems are designed to run on any of aclass of machines; the system must be configuredfor each specific computer siteSYSGEN program obtains information concerning

the specific configuration of the hardware systemBooting starting a computer by loading the kernelBootstrap program code stored in ROM that is ableto locate the kernel, load it into memory, and start its

execution

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System Boot

Operating system must be made available tohardware so hardware can start itSmall piece of code bootstrap loader , locates thekernel, loads it into memory, and starts itSometimes two-step process where boot block at fixedlocation loads bootstrap loaderWhen power initialized on system, execution starts at afixed memory location

Firmware used to hold initial boot code